Customer is having different period types defined in their system for the purpose of time registration. They generated weekly periods for the defined types at the beginning of this year for a whole year. They have recently noticed that two days were skipped in December last week period, 30-31 December.

The only way for adding the missing days back at the period is to Rename the Record Info of 29.12 from 29.12 to 31.12 in Organization administration>Setup>Calendars>Period types and PMA>Setup>Timesheet>Timesheet period types. After both steps are done you should click Update worker periods first in Period types and after that on Timesheet period types.
This changed the week from ending on 29th to ending on 31st and this way the missing days were populated again.

However, you need Admin rights to do so and it's not an easy solution for user which are not Admins.
Client would like this functionality to be reviewed and it should be possible to make correction from user interface if one of the periods has not been generated properly.